Uterine contraction detection

ABSTRACT

A system for tracing uterine contraction is disclosed. The system comprises a probe and an electronics module. The electronics module includes a drive circuit and an output section having a frequency counter. The output section also includes a strip-chart recorder driven by a frequency-to-voltage converter. The probe includes an electro-mechanical transducer which is driven at resonance by the action of the drive circuit. When the probe contacts the abdomen of a pregnant subject, changes in the mechanical impedance of bodily tissue during a contraction affect the resonant frequency of the probe and thus the frequency output by the drive circuit. This frequency can be read directly from the frequency counter and recorded on the strip-chart so that contractions can be traced.

Cross Reference to Related Application(s)

This is a continuation of copending application(s) Ser. No. 07/234,437filed on Aug. 19, 1988, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a system for detecting changes inmechanical impedance and, more particularly, for tracing uterinecontractions.

Uterine contraction, although a gross mechanical phenomenon, has provedawkward to measure. Typically, uterine contractions are traced bymeasuring the spring resistance to a tocotransducer probe which pushesagainst the maternal abdomen, as disclosed in Hewlett-Packard,"Cardiotocograph", Application Note 700 F., 1979. The probe is typicallyheld in place and against the maternal abdomen by an elastic belt. Whencontractions occur, the probe encounters a more resistive medium and itmoves orthogonal to the abdominal surface. This method generallysuffices to trace uterine contraction. However, it is awkward toposition the elastic belt and probe properly. Furthermore, the pressureof the elastic belt and probe are a source of discomfort to the subject.

Another widely employed method of tracing contractions is even moreintrusive. Intrauterine measurement of contractions can be performedusing a balloon-tipped or open-ended fluid filled catheter, as disclosedby D. O. Thorne, I. Assadi, J. Flores, and J. Seitchik, "Therelationship of the maximum amplitude and the maximum and minimum slopeof the intrauterine pressure waveform in late pregnancy and labor", IEEETransactions on Biomedical Engineering., vol. BME-19, p. 388, 1972.

More recently, changes in the electrical activity of tissue duringcontractions have been employed in tracing contractions, as disclosed byC. Marque, J. M. G. Duchene, S. Leclercq, G. S. Panczer, and J.Chaumont, in "Uterine EHG Processing for Obstetrical Monitoring", IEEETransactions on Biomedical Engineering, Vol. BME-33, No. 12, December1986. The electrical activity is recorded as a electromyogram, alsoreferred to as a "electrohysterogram" or "EHG".

EHG measurements can be made using intrauterine or abdominal electrodes.Electrical signals at several frequencies have been observed tocorrelate with contractions. The main problems with EHG measurements arethat the signals are not strong, so that they are easily interfered withby electrical signals from spurious physiological activity, and thattheir correlation with contractions is not very strong. Furthermore,intrauterine electrodes are more intrusive than desired. Abdominalelectrodes are less intrusive, but pick up electrical activity resultingfrom other sources, such as skin stretching, respiratory movements, andmovement of abdominal muscles. Because of the weakness of the electricalsignals being monitored and the susceptibility of the signals to noisefrom sources other than the contractions of interest, the sensitivityand validity of abdominal EHG measurements are limited.

What is needed is an improved system and method for tracing uterinecontractions which provides accurate tracing and which is alsonon-intrusive, comfortable and easy to use.

SUMMARY OF THE INVENTION

In accordance with the present invention, changes in mechanicalimpedance are traced by monitoring their effect on the resonantfrequency of a mechanically oscillating system. Since intrauterinecontractions are inevitably accompanied by changes in the mechanicalimpedance of abdominal tissue, they can be traced using this system andmethod.

The mechanically oscillating system comprises a probe, or othertransducer assembly, including a mechanical interface and anelectro-mechanical resonant transducer. The resonant transducer has aresonant frequency at which it vibrates preferentially; this resonantfrequency varies with the mechanical impedance of a mechanicallyinterfaced body. The mechanical interface can be simply a surface whichcan be adhered or otherwise positioned against an abdomen or other bodyof interest. This surface can be part of a probe housing which enclosesthe electro-mechanical transducer.

The electro-mechanical transducer can be implemented by taking advantageof a variety of phenomena, including the piezo-electric effect andelectro-magnetic effects. In the latter case, the transducer can includea permanent magnet, an electro-magnet, and a spring for regulating theirrelative positions. An electric signal applied to the electro-magnetcauses relative displacement of the magnets. By rigidly attaching one ofthe magnets to the mechanical interface, the latter can be made todisplace adjacent tissue.

The transducer has a resonant frequency which is a function of themechanical impedance of the spring and effective masses and mechanicalimpedances associated with each magnet. When the mechanical interface isadhered to an abdomen, the resonant frequency varies sensitively withthe "spring constant" of the abdominal tissue. Preferably, thetransducer is designed so that the range of resonant frequencies isspanned by the "low sonic" frequencies, i.e., 20 Hz to 200 Hz.

Resonant frequency is monitored using a feedback circuit which drivesthe transducer at its resonant frequency. The output of the feedbackcircuit is then a signal at the current resonant frequency of thetransducer. By tracking the frequency of this output, changes inresonant frequency, and thus changes in the mechanical impedance of anabdomen, and thus, contractions can be monitored. The frequency of thefeedback circuit output can be monitored using a frequency counter anddisplay and/or using a frequency-to-voltage converter to drive astandard stripchart recorder.

A major advantage of the present invention is that it employs an activemeasurement system. Instead of relying on the subject to provide theenergy, whether mechanical or electrical, for the measurement system,the present invention supplies a signal which is merely modulated by thesubject. In the preferred embodiment, the parameter being monitored isfrequency. Frequency is easily measured with precision and is lessvulnerable to interference than alternative parameters. In contrast, EHGsystems depend on the passive detection of a weak electrical signalwhich is subject to interference by other electrical signals. Also,since contractions inevitably are accompanied by significant changes inmechanical impedance, there is no doubt as to the validity of theparameter being measured. Conveniently, the present invention disposesof the elastic belt required of typical abdominal toco-transducers.Consequently, measurement is less effected by shifting of such a beltwhen the subject shifts body position.

Thus, the present invention provides a system and method for tracinguterine contractions which is convenient, accurate and reliable.Furthermore, it is readily apparent that the present invention isapplicable to the measurement of changes in mechanical impedanceassociated with other tissue changes and to a wide variety of othersubjects, not necessarily living, in which changes in mechanicalimpedance are of interest. These and other features and advantages ofthe present invention are apparent from the description below withreference to the following drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of a system for tracing uterine contractionsin accordance with the present invention.

FIGS. 2A and 2B are plan and sectional views, respectively, of a probeof the system of FIG. 1 with its cover removed.

FIG. 3 is a circuit diagram of a drive circuit of the system of FIG. 1shown in relation to the probe of FIGS. 2A and 2B.

FIG. 4 is a schematic diagram of a mechanical model of the probe ofFIGS. 2A and 2B.

FIG. 5 is a block diagram of an alternative uterine contraction tracingsystem in accordance with the present invention.

In the figures, for elements referred to by a three-digit referencenumeral, the first digit of the reference numeral is the figure numberin which that element is first depicted.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, a uterine contraction tracingsystem 100 comprises a probe 101 and an electronics module 103. Probe101, can be attached to a body 99 with medical-grade, double-sidedadhesive tape 105. Electronics module 103 includes a drive circuit 107and an output section 109. Probe 101 and drive circuit 107 areelectrically coupled by a cable 110 and constitute an oscillator. Outputsection 109 includes a frequency display 111 and a frequency-to-voltageconverter 113 for driving a strip chart recorder 115.

Probe 101 incorporates a electro-mechanical ("E/M") resonant transducer117, i.e., a transducer which has a resonant frequency enclosed by ahousing 119 and a cap 121. A surface 120 of housing 119 serves, alongwith tape 105, as a mechanical interface with body 99. When driven by acyclical drive signal, transducer 117 can cause probe 101 to vibrate.While not attached to a substantial object, probe 101 can be made tovibrate at a free-space resonant frequency by drive circuit 107. Whileattached to a substantial object, probe 101 is driven at a differentresonant frequency. The instantaneous value of this frequency is afunction of the mechanical impedance of body 99. Thus, by monitoringthis resonant frequency, the mechanical impedance of body 99 can betraced. Since, contractions are accompanied by changes in mechanicalimpedance, they can be detected and characterized using uterinecontraction tracing system 100.

Transducer 117 comprises a permanent magnet 223 and an electro-magnet225, shown in FIGS. 2A and 2B. Permanent magnet 223 is bonded at thefree end 227 of a leaf spring 229 cantilevered from a support post 231rigidly attached to housing 119. Electro-magnet 225 comprises a plasticbobbin 233 and a conductive coil 235 which surrounds permanent magnet223 as shown in FIG. 2B. Coil 235 has a signal lead 237 and a groundlead 239 which extend through cable 110 to electronics module 103.

The degree to which permanent magnet 223 extends into electro-magnet 227depends on the forces applied by the latter and by leaf spring 229. Acyclical electric signal through coil 235 causes permanent magnet 223 tooscillate relative to the rest of probe 101, causing housing 119 tovibrate. These vibrations can be damped by body 99 when attached by tape105 to the mechanical interface surface 120 of housing 119. This dampingaction determines the degree to which the frequency of the vibrationsdiffer from the free-space resonant frequency of probe 101.

Drive circuit 107 is designed to drive probe 101 at its resonantfrequency, as modified by the mechanical impedance of body 99 whenattached. Drive circuit 107 comprises a differential amplifier 341, apositive feedback resistor 343, a negative feedback resistor 345 and anAC-coupling capacitor 347, as shown in FIG. 3. Lead 237 from coil 235(depicted in FIGS. 2A and 2B) of probe 101 is coupled to node A of drivecircuit 107, while lead 239 is coupled to node D, which is at ground.Node A is coupled to the "+" input of differential amplifier 341. The"-" input of differential amplifier 341 is AC-coupled to ground viacapacitor 347.

The output of differential amplifier 341 at node C is fed back to node Aand thus to "+" terminal of amplifier 341 via a positive feedback loop349 including resistor 343. The output at node C is also fed back tonode B and thence to the "-" input of amplifier 341 via a negativefeedback loop 351 including resistor 345. In the illustrated embodiment,positive feedback resistor 343 is nominally 100 kΩ, negative feedbackresistor 345 is nominally 500 kΩ, and capacitor 347 is nominally 10 μF.By constrast, the DC resistance of the transducer is of the order of 5Ω.At the resonant frequency of the transducer, the voltage swings at the"+" terminal of amplifier 341 due to the positive feedback signal arelarger than the voltage swings at the "-" terminal of amplifier 341 dueto the negative feedback signal. Thus, although the amplifier is stablybiased, the system oscillates at the resonant frequency.

The amplifier output not only drives probe 101 according to the signalat node A, but also provides via node C a buffered signal at node E,shown in FIG. 1, of output section 109 for driving frequency display 111and frequency-to-voltage converter 113 for driving strip-chart recorder115.

The action of probe 101 can be further understood with reference themechanical model of FIG. 4. Probe 101 can be represented by a mass M₁,corresponding collectively to housing 119, cap 121 and electromagnet225, a mass M₂ corresponding to permanent magnet 223, and a spring force453, corresponding to leaf spring 229. In addition, mechanicalresistance of probe 101 is represented by dashpot 455. Body 99 ismodelled by an infinite rigid mass 457, variable spring 459 and dashpot461. (In a more complete model, M₁ would include a componentrepresenting bodily tissue displaced by the action of probe 101). Thespring constant of variable spring 459 varies with changes in mechanicalimpedance of body 99 which occur during contractions.

When not attached to body 99, probe 101 has a free-space resonantfrequency of:

    f.sub.fs =[K(M.sub.1 +M.sub.2)/(M.sub.1 M.sub.2)].sup.1/2.

If attached to a rigid body of infinite mass, probe 101 would have aresonant frequency of:

    f.sub.∞= (K/M.sub.2).sup.1/2.

The preferred range of oscillation frequencies is from about 20 Hz to 80Hz. F_(fs) for the illustrated embodiment is about 50 Hz. In the casesof interest, the resonant frequency is intermediate between f_(fs) andf.sub.∞ and varies with the mechanical impedance of body 99.

In uterine contraction tracing system 100 described above, changes inresonant frequency are detected by driving transducer 117 at resonantfrequency and monitoring the drive frequency. In an alternative uterinecontraction tracing system 500, shown in FIG. 5, a drive signal ismaintained at a constant frequency and changes in resonant frequency aredetected by monitoring the phase relationship between the current andthe voltage across a transducer 501.

System 500 comprises transducer 501, an AC voltage source 503, aresistor 505 and a phase meter 507. Transducer 501 and resistor 505constitute a transducer assembly. Voltage source 503 drives transducer501 at a constant frequency. Phase meter 507 detects the voltage acrosstransducer 501 via lines 509 and 511 and the voltage across resistor 505via lines 513 and 515 and measures the phase differential between thetwo voltages. The voltage across resistor 505 is in phase with thecurrent through transducer 501 so that the phase differential measuredby phase meter 507 is the difference between the phases of the voltageand current through transducer 501. In order for the transducer 501 tobe driven at a constant frequency, the phase meter 507 is connected tothe voltage source 503 via feedback loop 520.

When transducer 501 is interfaced with a body and the mechanicalimpedance of that body changes, the resonant frequency of transducer 501changes. Because AC voltage source 503 drives transducer 501 at aconstant frequency, the difference between the drive frequency and theresonant frequency changes with resonant frequency. This frequencydifference results in a phase difference measured by phase meter 507.Thus, phase meter 507 can be used to indicate changes in mechanicalimpedance, for example, such as those caused by uterine contractions.

An alternative embodiment employs a transducer with two electromagnets,rather than one electromagnet and one permanent magnet. Anotherembodiment uses a transducer fabricated by modifying an small,inexpensive loudspeaker by cutting away portion of the cone and addingmass to lower its resonant frequency to about 50 Hz. Also, when a bodyis appropriately positioned, the incorporated probe can be held inposition by gravity so that tape is not required.

In the illustrated embodiment, contractions are detected because changesin mechanical impedance of the contracting tissues affect the frequencyoutput from a resonant circuit. However, the present invention providesfor non-resonant circuits as well. In an alternative embodiment, a motordrives an eccentric shaft, e.g., a shaft with an off-center weightattached. A constant current drives the motor at a constant rotationalrate. The eccentric shaft causes an attached frame to vibrate. The motorthus serves as a non-resonant electro-mechanical transducer. When theframe is pressed against a body, the rotation rate of the motor varieswith changes in the mechanical impedance of the body duringcontractions. Thus, one can monitor contractions by observing changes inthe motor rotation rate.

The foregoing discussion has focussed on frequency as the parameter ofinterest in detecting contractions. However, other parameters can beused. For example, changes in mechanical impedance can affect theamplitude of a current through an electro-magnetic resonator.

While contractions can be detected by examining the effect of changes inmechanical impedance on an electro-mechanical transducer, it is alsopossible to characterize contractions by their effect on the signalgenerated from such a transducer. During a contraction, there is achange in the effect of bodily tissue on signal amplitude and phase.These changes can be detected as the signal is received after beingtransmitted through or reflected by the body. One embodiment usespseudo-random numbers to generate a signal. An advantage of thispseudorandom approach is that the resulting signal is broad spectrum andthus minimally disturbing to a subject. In addition, such signals areeasily distinguished from noise, even where the latter's amplitude isrelatively high. Alternatively, the generated signal can be pulsesgenerated by a series of taps. The changing delays in the transmitted orreflected signal can be used to characterize a contraction.

In related embodiments, the tendency of a change in mechanical impedanceto change frequency can be compensated by changing another variable,e.g., drive current. In this case, contractions are not reflected infrequency changes since frequency is held constant. However,contractions can be traced by tracking the drive current required tomaintain constant amplitude or, in the eccentric shaft embodiment,rotation rate.

While tracing system 100 detects changes in mechanical impedance bymonitoring the drive signal, the present invention providesalternatives. For example, a separate acoustic receiver can be used todetect acoustic waves transmitted through or reflected by a body.

A uterine contraction system can incorporate multiple probes for currentdetection of changes in mechanical impedance at several locations on abody. Multiple probes cannot be used conveniently with the currentmethod, because multiple belts would be required. The present inventionprovides for a multiple probe system which can be used to monitor theprogress of single contractions. This monitoring can be used todistinguish contractions from localized false labor contractions whichcould confuse a single probe system. In addition, while a single probesystem might fail to detect contractions due to probe misplacement, amultiple probe system would be much less susceptible to this problem.

The present application is described above as applied to the detectionof uterine contractions. More generically, the principles involved inthe present invention include the modulation of signals by changes inmechanical impedance of a body. With appropriate modifications, thesystem and method of the present invention can be used to detect changesin mechanical impedance in tissue reflecting other physiological events,such as voluntary muscular activities. Furthermore, applications of thepresent invention are not limited to physiological phenomena, but caninclude measurement of tension, rigidity and viscosity where desired.These and other modifications and variations are provided for by thepresent invention, the scope of which is limited only by the followingclaims.

What is claimed is:
 1. A system for detecting uterine contractions inand adapted to be coupled to a body of a pregnant woman, said bodyhaving a variable mechanical impedance that varies with uterinecontractions, said system comprising:an electro-mechanical resonanttransducer including an interface wall having one side for contactingthe body and a second side disposed away from the body, and a flexiblespring means being connected to the second side of the interface wall;the transducer operating with a resonant frequency; a drive circuitelectrically coupled to said resonant transducer for providing acyclical drive signal thereto; driving means arranged for responding tothe drive signal so that the transducer is driven to vibrate at itsresonant frequency via the spring means; and monitor means for detectingchanges in the resonant frequency when the mechanical impedance of thebody varies with uterine contractions.
 2. The system of claim 1 whereinthe drive circuit further includes:a voltage source coupled to theresonant transducer for producing a voltage and thereby generating anelectrical current therein; means coupled to the spring means forresponding to the electrical current to drive the spring means intovibratory motion; and a phase meter coupled for measuring changes inphase between the voltage and the electrical current; wherein changes inthe mechanical impedance of said body are detectable as changes in saidphase.
 3. The system as recited in claim 2 wherein the interface wall isrelatively rigid.
 4. The system of claim 1 wherein the cyclical drivesignal has a parameter which varies with changes of the resonantfrequency of the transducer when it is in contact with the body; andsaidmonitor means includes means for detecting changes in said parameterwhen uterine contractions vary the mechanical impedance of the body. 5.The system as recited in claim 4 wherein the interface wall isrelatively rigid.
 6. The system of claim 4 wherein the parameter isdrive signal frequency and said monitor means is electrically coupled tosaid drive circuit for detecting changes in the drive signal frequency.7. The system as recited in claim 6 wherein the interface wall isrelatively rigid.
 8. The system as recited in claim 1 wherein theinterface wall is relatively rigid.
 9. The system as recited in claim 1wherein the drive circuit incorporates a feedback loop for driving thetransducer at its resonant frequency.
 10. The system as recited in claim1 further including a voltage source coupled to the transducer forproducing a voltage and thereby generating an electrical current as thedrive signal; and the means for detecting changes is operative forresponding to variances in the electrical current when uterinecontractions vary the mechanical impedance of the body.
 11. A method ofdetecting uterine contractions in a body of a pregnant woman, said bodyhaving a mechanical impedance which changes with the occurrence ofuterine contractions, said method comprising the steps of:driving atransducer to vibrate at a resonant frequency; connecting saidtransducer to the body so that the resonant frequency of the transducervaries with said mechanical impedance; and measuring the mechanicalimpedance of the body by sensing changes in resonant frequency of thetransducer when it is connected to the body.